The present invention relates to a substrate material and a method for separating oxide superconductors. More specifically, it relates to a substrate material for preparing a superior oxide superconductor film (either thick or thin), a material for an intermediate layer, or a substrate insulator for silicon on insulator (abbreviated as S.O.I.). More broadly, it relates to techniques useful when applied to the field of electronics including devices using Josephson junction as well as that of electric power including power storage and power transmission.
Various methods have been known for the manufacture of oxide superconductors, including a method of depositing a film using epitaxial growth of an oxide superconductor on a substrate. Also, various methods are known for forming films, such as molecular beam epitaxy (abbreviated as M.B.E.) and high-frequency spattering. An atmosphere in which a film is formed almost always includes oxygen. When a film of an oxide superconductor is formed according to these methods, a substrate material is required that does not react with the oxide superconductor. As substrate materials satisfying this condition, MgO, ZrO.sub.2, MgAl.sub.2 O.sub.4, LaAlO.sub.3, SrTiO.sub.3, LaGaO.sub.3, etc. are used. These materials are less reactive with oxide superconductors than Si or Al.sub.2 O.sub.3.
However, according to studies having been done so far in the field of superconductivity, it is known that substrate materials such as SrTiO.sub.3 and LaGaO.sub.3 show superior superconducting characteristics compared to MgO, ZrO.sub.2, MgAl.sub.2 O.sub.4, or LaAlO.sub.3 when they are used in devices.
Table 1 shows the lattice constants of the substrate materials and the oxide superconductors mentioned above, and Si and GaAs. In Table 1, the values of lattice constants of SrTiO.sub.3 and YBa.sub.2 Cu.sub.3 O.sub.y (y being approximately 7) shown with the "*" mark attached are those multiplied by the square root of 2 so as to emphasize lattice mismatching. As readily seen from Table 1, SrTiO.sub.3, LaGaO.sub.3 and the like have smaller mismatching of lattice constants with the oxide superconductors. The smaller the differences are in lattice constants between the substrate materials and the oxide superconductors, the easier the epitaxial growth becomes, so as to make it easy to obtain an oxide superconductor film of single crystal and thus improve the superconducting characteristics of the oxide superconductors in a film form.
As materials having smaller mismatching with oxide superconductors, perovskite-type compounds such as LaGaO.sub.3 have been used. FIG. 3 shows the unit crystal structure of LaGaO.sub.3 and an oxide superconductor YBa.sub.2 Cu.sub.3 O.sub.y. It should be noted that oxygen is not shown in FIG. 3. As shown in FIG. 3, many compounds of perovskite type are known to have lattice constants close to a multiple of the lattice constants of oxide superconductors or to 2n times the multiple (n being an integer).
The inventors of the present invention have studied the state of prior art described above and found the following problems.
The lattice constants of an oxide superconductor film prepared by M.B.E. or high-frequency spattering described above vary depending on the concentration of oxygen in the atmosphere during film formation or in a cooling process.
Also, the temperature during the formation of an oxide superconductor film by M.B.E. or high-frequency spattering is normally as high as 600.degree. C. to 800.degree. C. Therefore, there exits mismatching between a substrate material and an oxide superconductor caused by differences in the thermal expansion of the materials. As a result, because of the mismatching due to thermal expansion as well as that due to differences in the lattice constants, an oxide superconductor with superior superconducting characteristics cannot be obtained when the conventional substrate materials described above are used.
Furthermore, in order to obtain a device in which oxide superconductor and semiconductor elements coexist, an oxide superconductor film has to be prepared on, for example, an Si substrate. However, when a film of an oxide superconductor is prepared by the M.B.E. or the high-frequency spattering mentioned above, because the temperature of film formation is high, the oxide superconductor and the Si substrate react with each other, and the superconducting characteristics of the oxide superconductor may never materialize.
Also, an article entitled Thermal Analysis of Rare Earth Gallates and Aluminates (H. M. O'Bryan et al., J. Mater. Res., vol. 5, No. 1, pp. 183-189) discloses the fact that LaGaO.sub.3 crystal undergoes a phase transition between orthorhombic and rhombohedral structures at a temperature of about 145.degree. C. (see FIGS. 1 and 2, and the last paragraph on page 184) and that NdGaO.sub.3 crystal is orthorhombic from 25.degree. C. and up to 1000.degree. C. (see Table 1).